The invention is related to an improved rotational moulding process of ethylene polymers or copolymers and to ethylene polymers having a controlled particle size and morphology. The invention is also related to a method for the manufacture of polymers, which are useful in such applications, where proper particle size and good morphology are mandatory requirements.
Rotational moulding is a process where hollow plastic articles can be produced. In the process very fine plastic powder is loaded into a slit, hollow metal mould. Then the mould is rotated round the equatorial and polar axes while the polymer powder is heated above its melting point. When the melted plastic covers the inside surface of the mould, the assembly is cooled to room temperature and the part removed.
The properties of the final article can be influenced by the choice of the plastic material used as raw material, the plastic powder characteristics and the process parameters, such as heating temperature, heating time, cooling time and other conditions. Especially the polymer powder characteristics are very important parameters.
Good moulding resins for rotational moulding resins should satisfy, among others, the following criteria:
The granules must be free flowing, substantially spherical in shape and free of any tails or hairs, which could decrease the flowability of the resin,
The particle size should be relatively small and the particle size distribution of the polymer particles should be relatively narrow, and
The bulk density of the granules should be high to provide good flowability and close compaction in the mould.
According to the conventional prior art the polymers for rotational moulding have been prepared by first melt compounding the polymerization reactor product and by pelletizing to granules. Additives are often melt compounded with the polymer in this step. Then the pellets are ground and classified to proper particle size. The pelletizing and grinding operations are costly because of high energy consumption, and they frequently produce also irregularly shaped particles, which impede flowability and reduce bulk density.
Polymer products obtained by using Ziegler-Nata catalysts contain very much fine particles (xe2x80x9cfinesxe2x80x9d), which may cause dust explosions and are thus not suitable for rotational moulding. The use of reactor xe2x80x9cfluffxe2x80x9d coming out from the polymerization reactor has also been studied for the rotational moulding process. However this typically produces polymer particles having too large particle size and wide particle size distribution. Therefore it has not been possible to produce good polyethylene resins for rotational moulding process in this way. Another solution to avoid the problems above has been disclosed in U.S. Pat. No. 4,408,859.
According to this patent the polymer fluff is blended with additive materials for making a masterblend, this masterblend is mixed in an intensive mixer with additional thermoplastic granules below the softening point and the mixing is continued until at least 80% of the granules are smaller than about 30 mesh and the bulk density has been increased by at least 10% over unfinished granules. Thereafter the mixture is allowed to cool and a substantial part of the particles larger than 30 mesh is removed. Though eliminating the melt compounding, pelletizing and grinding steps according to the conventional technique, this method only replaces these steps with other energy consuming operations and does not make it possible to manufacture polymers for rotational moulding process straight in the polymerization reactor without any additional need to reduce the polymer particle size.
The object of the invention is a rotational moulding process of ethylene polymers or copolymers having a controlled particle size and morphology. The polyethylene resin is produced in a polymerization reactor in such form, that it can be used without any additional operations for rotational moulding processes, where controlled particle size and good morphology are essential. Another object of the invention is to provide a polymerization process, which produces in particle form such polyethylene, which is applicable for rotational moulding process without need for additional pelletizing, grinding or intensive mixing steps.
According to the invention it has been found that the objects above can be achieved by carefully making certain selections related to the catalyst type, catalyst particle size and particle size distribution.